#include <cstdlib>
#include <vector>

using namespace std;

struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

class Solution {
public:
    TreeNode* mirrorTree(TreeNode* root) {
        if (root == nullptr) {
            return root;
        }
        mirrorTree(root->left);
        mirrorTree(root->right);
        swap(root->left, root->right);
        return root;
    }
};

